The present invention relates to a casing structure for a turbojet engine combustion chamber, more particularly, such a turbojet engine utilized in aircraft with free turbines or dual, counter-rotating "prop fans".
In turbojet engines presently in use, air may be tapped from the combustion chamber in order to supply various needs in the aircraft, such as cabin pressurization. It is necessary to tap the air from the combustion chambers when the air that is tapped from a compressor upstream of the combustion chamber no longer sufficies for adequate pressurization. This circumstance is most often encountered when the aircraft is descending from high altitude.
The mechanism utilized to tap the air from the combustion chambers must be such that the air is not mixed with fuel in suspension. This is carried out in known engines by air-tap pipes which receive air from a chamber located between the exit of the high pressure compressor and the entrance to the combustion chamber proper. A typical prior art layout is illustrated in FIG. 1 in which the air flow from a tap location upstream of the diffuser 1 is illustrated by the arrows. A forward end of the combustion chamber has a frusto-conical collar 2 which, along with upstream collar 3, defines an air-tap passage. The collar 3 has a connecting flange 4 which is attached to the outlet end of the high pressure compressor, while a fastening flange 5 interconnects the upstream end of the combustion chamber to an outer casing.
This known system suffers from several significant drawbacks. First, the fuel injectors must be located downstream of the collar 2 so as to avoid contamination of the tapped air and must also extend into the upstream end of the combustion chamber. This requires that they be formed in a complex elbow shape to ensure that the fuel injection takes place in the proper location in the combustion chamber, a shape which serves to hamper the assembly of the structure.
Second, this design unduly increases the overall length of the engine, since the air-tap structure must be interposed between the compressor and the combustion chamber. In order to provide the maximum usable air frame space, it is desirable to minimize the dimensions of the turbojet engine as much as possible.